Process for reading fractions of intervals between contiguous photo-sensitive elements in a linear optical sensor

ABSTRACT

In a process for reading fractions of an interval between contiguous photo-sensitive elements in a linear optical sensor, of a type used in a goniometer, an angle measured is an angle formed with a reference axis of the goniometer, perpendicular to the linear optical sensor, by a light beam which is trained on the optical sensor by an optical device. The process comprises: a reading of a current image constituted by an order totality of intensities of incident radiations read on contiguous photosensitive elements; processing of data taken from the current image by means of a process which converges towards a result defining, with respect to an origin determined by an intersection of the reference axis with an axis of the sensor, a distance d of a point of incidence on the sensor of an ideal optical axis of the light beam.

RELATED APPLICATIONS

[0001] This application is a continuation of copending application Ser.No. 10/122,182, filed Apr. 16, 2002.

BACKGROUND OF THE INVENTION

[0002] The fractions of the title relate to those existing in theinterval between photo-sensitive elements of the type used in agoniometer for measuring characteristic angles of motor vehicle wheels.

[0003] In these types of goniometers, the angle measured is the angleformed with the reference axis of the goniometers (which isperpendicular to the optical sensor and which defines an origin for themeasurement of the distances in a longitudinal direction) by a lightbeam which is trained on the optical sensor by an optical device.

[0004] The light beam is constituted by parallel rays which aregenerated by a light located at a sufficiently large distance withrespect to the distance between the optical device (cylindrical lens orslit) and the linear sensor.

[0005] When a cylindrical lens is used as the optical device, the focallength of the lens is equal to the distance between the lens and thesensor. In this case all the parallel rays which strike the frontsurface of the lens are concentrated in a line which intersects thelinear sensor in a very precise zone which is conditioned not only bythe exactness of the lens positioning but also by the angle to bemeasured which is the angle formed by the incident light beam with theline perpendicular to the linear sensor. It is obvious that as the anglebecomes the greater so do the alterations of the precisely-directedlight hitting the sensor, with the immediate consequence that alteredimages are produced by the sensor, resulting in a progressive increasein the lack of precision of the measurement.

[0006] The phenomenon is progressively more accentuated the smaller thefocal distance and the greater the angle to be measured.

[0007] One way of reducing this phenomenon is to use a corrector opticalgroup in association with the sensor. This however leads to considerablecomplications in construction as well as higher costs.

[0008] A further consideration is that the resolution of the linearsensor is physically connected to the distance between onephotosensitive element and an adjacent one.

[0009] This means that according to known realizations it does not seempossible to determine a distance from the origin which is not equal to awhole multiple of the interval (constant) between element and element.Thus it does not seem possible to read fractions of this interval.

[0010] A similar imprecision would occur should a light beam becollimated with a transversal dimension able to generate a light spotwhich is smaller than the dimensions of a photosensitive light element.The result could be that the linear sensor may not even be activated inall those cases where the light beam did not strike any of thephotosensitive elements.

[0011] The possible solution, of increasing the threshold of sensitivityby increasing the number of photosensitive elements per unit of lengthof the linear sensor, or by considerably reducing the interval betweenone photosensitive element and another, is at present so expensive as tobe impracticable.

[0012] In any case a greater goniometer resolution for measuring thecharacteristic angles of a motor vehicle's wheels using linear sensorstogether with a greater width in the field of measurement is anestablished need in the field.

[0013] The main aim of the present invention is to obviate thelimitations and drawbacks in the prior art.

[0014] An advantage of the invention is that it does not introduce anyspecial modifications, from the constructional point of view, to theapparatus used. These aims and advantages and others besides are allachieved by the present invention, as it is characterized in the claimsthat follow.

SUMMARY OF THE INVENTION

[0015] In a process for reading fractions of an interval betweencontiguous photo-sensitive elements in a linear optical sensor, of atype used in a goniometer, an angle measured is an angle formed with areference axis of the goniometer, perpendicular to the linear opticalsensor, by a light beam which is trained on the optical sensor by anoptical device. The process comprises: a reading of a current imageconstituted by an ordered totality of intensities of incident radiationsread on contiguous photosensitive elements; processing of data takenfrom the current image by means of a process which converges towards aresult defining, with respect to an original determined by anintersection of the reference axis with an axis of the sensor, adistance d of a point of incidence on the sensor of an ideal opticalaxis of the light beam.

BRIEF DESCRIPTION OF THE DRAWINGS

[0016] Further advantages of the present invention will better emergefrom the detailed description that follows of a preferred butnon-exclusive embodiment of the invention, illustrated purely by way ofnon-limiting example in the accompanying figures of the drawings, inwhich:

[0017]FIG. 1 is a schematic plan view of a measurement sensor applied toa wheel of a motor vehicle;

[0018]FIG. 2 is a diagram of a goniometer equipped with the sensor ofFIG. 1;

[0019]FIG. 3 is a digital schematic illustration showing the progressivesignal image formed on the linear sensor;

[0020]FIG. 4 shows a possible electronic diagram for processing thesignals from the linear sensors;

[0021]FIG. 5 is a graph relating to the generation of synchrony for theanalog-digital converter;

[0022]FIG. 6 shows the ratio between the synchronization signal and theimage generated by the linear sensor;

[0023]FIG. 7 shows, in much-enlarged scale, the phase/time ratio betweenthe synchronization signal of the analog-digital converter and thesignal at the output of the goniometer linear sensor amplifier.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0024] With reference to FIGS. 1-7 of the drawings, 1 denotes in itsentirety a measuring sensor applied on a rim of a wheel 2 of a motorvehicle; the sensor 1 is for determining the characteristic angles ofthe wheel. The measuring sensor 1 comprises two optical goniometers 3and 4 which are correlated with corresponding optical goniometersapplied on three more measuring sensors mounted on the remaining threewheels of the motor vehicle.

[0025] Referring to FIGS. 2 and 4, each opital goniometer isschematically constituted by a linear image optical sensor 5 whichcomprises a line of photo-sensitive elements 6 and by an optical device7 which has the task of training a light beam 8 on a linear sensor 6 inthe direction the angular measurement is to be made. In the illustratedexample the light beam 8 is coming from a light associated to themeasuring sensor mounted on one of the wheels contiguous to the wheel onwhich the measuring sensor 1 is mounted.

[0026] The angle measured is the angle comprised between the directionof the light beam 8 and the goniometer reference axis 19 which isdefined as the axis of the optical device 7, perpendicular to the linearsensor 5 at its median point. The following are very schematicallydenoted: f′ is the distance, measured on the axis 19 between the opticaldevice 7 and the linear sensor 5 (corresponding to the focal distance inthe case of use of an optical device 7 constituted by a lens); dindicates the distance from the origin of the scale of distances definedby the linear sensor 5; α indicates the angle between the axis 19 andthe direction of the light beam 8.

[0027] The reading of the distances from the origin o which are notwhole multiples of the interval existing between one photosensitiveelement and another but which include fractions of the interval is doneusing a process which comprises:

[0028] reading off the current image constituted by the ordered whole ofthe intensities of the incident radiations registered on the nearbyphotosensitive elements (FIG. 3 is a diagram with the x-coordinatesshowing distances from origin o and the y-coordinates showing lightintensities read by single photosensitive elements);

[0029] processing of the data included in the current image by means ofa process converging towards a result which defines, with respect to anorigin determined by the intersection of the reference axis with theaxis of the sensor, the distance of the point of incidence on the sensorof an ideal optical axis of the light beam.

[0030] The distance is determined by means of an interpolation processon a measurement of distance based on a comparison of the current imagewith a known image previously acquired by means of a calibrationoperation (known as a pattern or template) which is made to run over thecurrent image to be compared there-with using a suitable measurementsystem.

[0031] In particular, in a first embodiment, with T_(i), i=1, i=1 . . ., n the totality of photosensitive elements constituting the template,and I_(i), . . . , m, m>n, the totality of photosensitive elementsconstituting the current image, a possible measurement formula is thesum of the distances element by element where the distance can be theEuclidean distance, the distance of the absolute value or other; thedistance in element k being:$S_{k} = \sqrt{\sum\limits_{i}\left( {I_{i} - T_{i - k}} \right)^{2}}$

[0032] using Euclidean measurements, or$S_{k} = {\sum\limits_{i}\left( {I_{i} \cdot T_{i - k}} \right)}$

[0033] using absolute measurement values. The fraction of intervalbetween two contiguous photosensitive elements is determined using aninterpolation obtained considering the local minimum of the intervalk+1, k−1 in the curve passing through the distances corresponding toelements k, k+1, k−1, i.e. the fraction of interval being determinableusing the ratio:$f = \frac{d_{\quad {k + 1}} - d_{\quad {k - 1}}}{2\left( {d_{\quad {k + 1}} - {2d_{\quad k}} + d_{\quad {k - 1}}} \right.}$

[0034] where f represents the fractional part of the position of thepoint of incidence of the ideal optical axis of the light beam.

[0035] In a second embodiment, with T_(i), i=1, . . . , n the totalityof photosensitive elements constituting the template, and I_(i), i=1, .. . , m, m>n, the totality of photosensitive elements constituting thecurrent image, a possible measurement formula is the correlation i.e.the sum of the products element by element between the current image andthe pattern determined in calibration with the standard ratio:$S_{k} = {\sum\limits_{i}{{I_{i} - T_{i - k}}}}$

[0036] and, normalized$S_{k} = \frac{\sum\limits_{i}\left( {I_{i} \cdot T_{i - k}} \right)}{\sqrt{\sum\limits_{i}I_{i}^{2}}}$

[0037] where the fraction of interval between two contiguousphotosensitive elements is determined using an interpolation obtainedconsidering the local minimum of the interval k+1, k−1 in the curvepassing through the distances corresponding to elements k, k+1, k−1,i.e. the fraction of interval being determinable using the ratio:$f = \frac{d_{\quad {k + 1}} - d_{\quad {k - 1}}}{2\left( {d_{\quad {k + 1}} - {2d_{\quad k}} + d_{\quad {k - 1}}} \right)}$

[0038] where f represents the fractional part of the position of thepoint of incidence of the ideal optical axis of the light beam.

[0039] The distance d of the point of incidence on the linear sensor canalso be determined using an interpolation process on the current imageby directly interpolating the template.

[0040] The same distance can also be determined through calculation ofsymmetries in the current image, determining the eventual centre ofsymmetry and/or centre of mass. This determination can be obtained usingexpressions of the following type:$p = \frac{\sum\limits_{i}{i \cdot I_{i}}}{\sum\limits_{i}I_{i}}$

[0041] where p is the position of the center of mass relating to thecurrent image. It is advisable to use some of the described methodstogether to minimize the errors introduced or amplified by eachindividual one.

[0042] During operation, in order to realize the analysis of the signalover more than one element the analog-digital conversion must besynchronized exactly with the scanning on the sensor, i.e. with theoperation with which a synchronization signal is used to analyze thesignal contents of each photosensitive element of the sensor struck bythe light trained on the optical element.

[0043]FIG. 4 shows an electronic data processing system for the signalfrom the above-described linear sensors. The signal of the linear sensor5 mounted on the goniometer 3 and the sensor 5 mounted on the goniometer4 are processed by a single signal processor circuit comprisingrespectively the amplifier-conditioner, the analog-digital converter 11and the synchronism generator circuit 12.

[0044] The signal processor circuit enables the synchronizationnecessary between the scanning of the signals produced by the singlephotosensitive elements of the linear sensors 5 and the analog-digitalconversion operation so as to enable an exact numerical calculation by asignal-calculation calculating circuit of each photosensitive element,so as to be able to process data concerning the angular valuecorresponding to the angle over which an interval between twoconsecutive photosensitive elements is observed.

[0045] The synchronism generator generates the signals needed for thefunctioning of the linear sensors, and in particular it generates thescanning synchronisms according to a known procedure, typical of linearimage sensors and not part of the present invention.

[0046] In particular the synchronism generator generates, synchronouslywith the scanning of a single photosensitive element, a RDAD signal;this signal informs the analog-digital converter of the instant t1 inwhich the conversion operation starts, i.e. the instant at which theavailable signal is surely available to the desired photosensitiveelement, and at which the signal is sufficiently stable (FIG. 5).

[0047]FIG. 5 shows (much-enlarged) the synchroniation signal 14 RDAD andthe signal at the output of the linear sensor (or rather at the outputof the relative amplifier-conditioner circuit) relative to thephotosensitive element, and the time t1 at which the converter beginsthe conversion. The converter, equipped with an internal multiplexer andsynchronized by the RDAD signal, analyzes alternatively the signal ofthe sensor of the goniometer 3 or 4 and makes available a Doutdigitalized signal; at the same time, at the end of the conversion, anAD irquinput signal is generated which a delay, realized using the samesynchronization generator 12, conditions and returns so as to inform thecalculation unit with the AD irqout signal that the data concerning thephotosensitive element is available.

[0048] The result is shown in digital format in FIG. 3, in which thecontents of the signal of the single photosensitive elements indicatedon the x-axis are given in binary form on the y-axis, limited to anenlarged zone around the image formed on the linear sensor.

[0049]FIG. 6 shows in less enlarged form the relation between thesynchronization signal 14 and the image 13 generated by the linearsensor; FIG. 7 gives a much-enlarged indication of the phase andtemporal ratio between the synchronization signal 14 of theanalog-digital converter and the signal 13 at the amplifier output 9 or10, respectively of the linear sensor of the goniometer 3 or 4.

[0050] Only by exact sychronization between the scanning operation ofthe signal on the single photosensitive elements of the linear sensorand the converersion operation is it possible to gather the data on thecontents of the signal of the nearby photosensitive elements in order toget the angular data with a greater whole element resolution, and thusdetermine the minimum angle requested.

What is claimed is:
 1. A process for reading fractions of an intervalbetween contiguous photo-sensitive elements in a linear optical sensor,of a type used in a goniometer, in which an angle measured is an angleformed with a reference axis of the goniometer, perpendicular to thelinear optical sensor, by a light beam which is trained on the opticalsensor by an optical device, comprising: reading a current imageconstituted by an ordered totality of intensities of incident radiationsread on contiguous photosensitive elements; and processing data takenfrom the current image by means of an interpolation process whichconverges towards a result defining, with respect to an origindetermined by an intersection of the reference axis with an axis of thesensor, a distance d of a point of incidence on the sensor of an idealoptical axis of the light beam.
 2. The process of claim 1, wherein thedistance is determined by means of an interpolation process on ameasurement of distance based on a comparison of the current image withan image previously acquired which is compared with the current imageusing a suitable measurement system.
 3. The process of claim 2, whereinT_(i), i=1, . . . , n, is the totality of photosensitive elementsforming a template, and I_(i), i=1, . . . , m, m>n, is the totality ofphotosensitive elements forming the current image, a possiblemeasurement formula is the sum of the distances element by element wherethe distance can be the Euclidean distance, the distance of the absolutevalue or other; the distance in element k being:$S_{k} = \sqrt{\sum\limits_{i}\left( {I_{i} - T_{i - k}} \right)^{2}}$

with Euclidean measurements; or$S_{k} = {\sum\limits_{i}{{I_{i} - T_{i - k}}}}$

using absolute measurement values; the fraction of interval between twocontiguous photosensitive elements being determined using aninterpolation obtained considering the local minimum of the intervalk+1, k−1 in the curve passing through the distances corresponding toelements k, k+1, k−1, i.e. the fraction of interval being determinableusing the ratio:$f = \frac{d_{\quad {k + 1}} - d_{\quad {k - 1}}}{2\left( {d_{\quad {k + 1}} - {2d_{\quad k}} + d_{\quad {k - 1}}} \right)}$

where f represents the fractional part of the position of the point ofincidence of the ideal optical axis of the light beam.
 4. The process ofclaim 2, wherein T_(i), i=1, . . . , n, is the totality ofphotosensitive elements forming a template, and I_(i), i=1, . . . , m,m>n, is the totality of photosensitive elements forming the currentimage, a possible measurement formula is the correlation i.e. the sum ofthe products element by element between the current image and thepattern determined in calibration with the standard ratio:$S_{k} = {\sum\limits_{i}\left( {I_{i} \cdot T_{i - k}} \right)}$

and, normalised:$S_{k} = \frac{\sum\limits_{i}\left( {I_{i} \cdot T_{i - k}} \right)}{\sqrt{\sum\limits_{i}I_{i}^{2}}}$

where the fraction of interval between two contiguous photosensitiveelements is determined using an interpolation obtained considering thelocal minimum of the interval k+1, k−1 in the curve passing through thedistances corresponding to elements k, k+1, k−1, i.e. the fraction ofinterval being determinable using the ratio:$f = \frac{d_{k + 1} - d_{k - 1}}{2\left( {d_{k + 1} - {2d_{k}} + d_{k - 1}} \right.}$

where f represents the fractional part of the position of the point ofincidence of the ideal optical axis of the light beam.
 5. The process ofclaim 1, wherein the distance is determined by means of a process ofinterpolation on the current image.
 6. The process of claim 1, whereinthe distance is determined by means of a process of interpolation of atemplate.
 7. The process of claim 1, wherein the distance is determinedby means of a calculation of symmetries in the current image, or of somedetails of the image, by determining a position of a centre of symmetryor a centre of mass; the determination being made by means ofexpressions of the following type:$p = \frac{\sum\limits_{i}{i \cdot I_{i}}}{\sum\limits_{i}I_{i}}$

where p is the position of the centre of mass relating to the currentimage.